A disc drive is a data storage device used to store digital data. A typical disc drive includes a number of rotatable magnetic recording discs that are axially aligned and mounted to a spindle motor for rotation at a high constant velocity. A corresponding array of read/write heads access tracks defined on the respective disc surfaces to write data to and read data from the discs.
Disc drive spindle motors are typically provided with a multi-phase, direct current (dc) brushless motor configuration. The phase windings are arranged about a stationary stator on a number of radially distributed poles. A rotatable spindle motor hub is provided with a number of circumferentially extending permanent magnets in close proximity to the poles. Application of current to the windings induces electromagnetic fields that interact with the magnetic fields of the magnets to apply torque to the spindle motor hub and induce rotation of the discs.
Accelerating a spindle motor from rest can be fraught with difficulty and involves a number of important considerations that must be adequately taken into account. First, it is important to accurately determine the rotational state of a disc drive spindle motor prior to application of drive signals to the motor. Application of drive signals to a spindle motor while the motor is in an unknown state could lead to the inadvertent rotation of the motor in the wrong direction. Rotating the spindle motor in the wrong direction, even for a very short time, can lead to premature failure of a disc drive; heads and disc surfaces can be damaged, and lubricating fluid used in hydrodynamic spindle motor bearings can be pumped out of the bearings.
Early disc drive spindle motor designs used Hall effect or similar external sensors to provide an independent indication of motor positional orientation. However, present designs avoid such external sensors and instead use electronic commutation and back electromotive force (bemf) detection circuitry to provide closed-loop spindle motor control, such as discussed in U.S. Pat. No. 5,631,999 issued to Dinsmore. Such approach generally entails applying a predetermined sequence of commutation steps to the phase windings of the spindle motor over each electrical revolution (period) of the motor. A commutation step involves supplying the motor with current to one phase, sinking current from another phase, and holding a third phase at a high impedance in an unenergized state.
Detection circuitry measures the bemf generated on the unenergized phase, compares this voltage to the voltage at a center tap of the windings, and outputs a signal at a zero crossing of the voltages; that is, when the bemf voltage changes polarity with respect to the voltage at the center tap. The point at which the zero crossing occurs is then used as a reference for the timing of the next commutation pulse, as well as a reference to indicate the position and relative speed of the motor. Although a center tapped motor is used for discussion purposes, non-center tapped motors are applicable as well.
Above an intermediate operational speed, the control circuitry will generally be able to reliably detect the bemf from rotation of the spindle motor, and will further be able to use the detected bemf to accelerate the motor to a final operational velocity. Below this intermediate speed, however, closed-loop motor speed control using detected bemf generally cannot be used since the spindle motor will not generate sufficient bemf at such lower speeds.
Thus, a related difficulty encountered in accelerating a disc drive spindle motor from rest is getting the motor to properly and safely rotate up to the intermediate velocity so that the closed-loop motor control circuitry can take over and accelerate the motor up to the operational velocity.
Several approaches have been proposed in the prior art to accelerate a disc drive spindle motor from rest to an intermediate velocity, such as exemplified by U.S. Pat. No. 5,117,165 issued to Cassat et al. This reference generally discloses determining the electrical rotational position of a spindle motor to determine the commutation state of the motor; that is, to determine the appropriate commutation pulses that should be applied to accelerate the motor based on the then-existing motor position. Drive pulses of fixed duration are applied to the motor to induce torque and initiate rotation of the motor, and the electrical rotational position of the motor is measured between application of each successively applied, fixed duration pulse.
Once the motor rotates sufficiently to induce a change in commutation state, the next set of drive pulses are applied, and position measurements are taken between the application of each set of the drive pulses as before. As the motor achieves a higher rotational velocity due to the successive “nudging” provided by the drive pulses, the time between successive commutation states becomes shorter, decreasing the number of drive pulses applied during each commutation state.
The intermediate velocity must be high enough to enable a hand off to the motor control circuitry; that is, the intermediate velocity must be high enough to enable the spindle motor to generate bemf that can be detected and used by the bemf detection circuitry. Sufficient bemf allows frequency lock by the motor control circuitry. This is at least equally important in non-center tapped motors since operation at a lower RPM makes it more difficult to obtain sufficient bemf.
Once the hand off to closed loop control has taken place the control circuitry allows the motor to spin for a short time to allow a phase lock oscillator (PLO) to settle to a frequency. The motor is also accelerated to increase the bemf. This is followed by a coast mode that allows a comparator to determine whether the motor is rotating at a speed corresponding to the PLO frequency. At this point an incorrect rotation speed results in a motor startup retry.
The delays in executing a motor retry after the hand off to closed loop operation can significantly degrade performance and can result in significant damage to the spindle motor. Waiting for the PLO to settle, accelerating the motor to a point of high bemf and comparing the bemf to the PLO frequency can take a significant amount of time. Also, delays in restarting a backward rotating spindle motor can allow this rotation to last more than a harmless period of time and result in damage to the bearings and other internal portions of the disc drive. If the resulting backward rotation causes fluid to leak from the bearings, contamination can result within the sealed environment of the disc drive.
Accordingly, there is a need for improvements in the art whereby a high performance spindle motor can be reliably evaluated during startup for incorrect speed and insufficient frequency lock.